Input devices based on touch screens provide a convenient method for inputting commands in a manner that is easily reconfigured. In such devices, a simulated button pattern is displayed on the screen and the user selects a “button” by touching the screen over the button image.
One particular type of touch screen makes use of frustrated total internal reflection in a light guide to detect when the screen has been touched. This type of touch screen is constructed from a touch panel, an image generator and an imaging system. The image generator creates an image of simulated buttons or other objects to touch. This image is visible through the touch panel. The touch panel includes an optically transparent layer, or light guide, having an index of refraction greater than that of the surrounding air. A light source generates a light signal that is reflected between first and second sides of the optically transparent layer. The imaging system records an image of a second side of the touch screen. When a user touches a first side of the touch screen, a portion of the total-internal reflected light is reflected towards the second side of the touch panel at an angle less than the critical angle and escapes from the second side of the touch panel. This creates a bright spot on the surface of the second side of the touch panel and the location of this bright spot is recorded by the imaging system. A simulated button push is generated based on the location of the detected bright spot.
One example of this type of touch screen is described in the International Application Published Under The Patent Cooperation Treaty (PCT) by the applicant “Konin-Klijke Philips Electronics N.V.”, having International Publication Number WO 2005/029394 A2 and published on 31 Mar. 2005 (hereinafter referred to as “Philips”) and entitled “Light Guide Touch Screen”. As illustrated in FIG. 1, Philips includes a light guide 102 arranged adjacent to a screen 101. The light guide 102 has a light source 108 arranged to emit light 110 into the light guide 102. The optical matching between the light guide 102 and its surroundings is adapted such that the light 110 of the light source 108 is normally confined within the light guide 102 by means of total internal reflection. However, a user establishing physical contact with the light guide 102 perturbs the state of total internal reflection, and some of the light 110 is extracted from the light guide 102. In the display device, light detecting means 103 is arranged to detect the light 110 and relate this detection to an input position where the user contact occurred.
Another example of this type of touch screen is described in U.S. Patent Application Publication US 2004/0108990 A1 to Lieberman et al. (hereinafter referred to as “Lieberman”) which shows a similar touch screen making use of frustrated total internal reflection to determine an input position where user contact to the screen occurred. As illustrated in FIG. 2, Lieberman employs a transparent data entry-object engagement surface 290, exhibiting total internal reflection. A planar beam of light, designated by reference numeral 299, is emitted by an illuminator 294 and coupled to an edge 295 of the surface 290 through which a beam 292 passes by total internal reflection. The presence of an object, such as a data entry object 296 in contact with the surface 290, causes light from the beam 292 to be scattered into a scattered beam 297 due to frustrated total internal reflection and inter alia to pass through the transparent data entry object engagement surface 290 so as to be detected by a camera 298.
A problem with these “frustrated total internal reflection” based touch screens is that it can be difficult to detect a “bright spot” over the noise created by the ambient light. In a bright environment, ambient light can pass through the touch screen to the light detector causing “noise” which makes the “bright spot” seem relatively dim.
As mentioned above, these touch screens are often used with an image generator which creates an image of simulated buttons or other objects to touch. The image generator often requires lighting and this lighting can also cause “noise”, making it more difficult to detect the “bright spot”.
One solution to these problems is to use a brighter light source for the light guide, however, this requires greater electrical power which is undesirable for portable devices.
It would be desirable to provide a “frustrated total internal reflection” based touch indication screen that uses less electrical power and works well even in bright ambient light environments.